1. Field of the Invention (Technical Field)
The present invention relates to a high power electric switch which has an ultra short rise time and can be fired at a repetition rate from less than a pulse per second to more than 20,000 pulses per second and can switch joules to megajoules of energy per pulse with switch rise times of less than a nanosecond, yet switch pulse widths ranging from picoseconds to milliseconds.
2. Background Art
Note that the following discussion is given for more complete background of the scientific principles and is not to be construed as an admission that such concepts are prior art for patentability determination purposes.
Large scale pulse power systems, such as accelerators, fusion accelerators, medical accelerators, high power microwave systems, and other high voltage or pulse power systems require the switching of very high power (megawatt) loads, for example from one Joule to megajoules per pulse, and high repetition rates, for example from less than one pulse per second to 20,000 pulses per second. Early studies at moderate pressures have shown breakdown strength in liquids to be a function of pressure up to at least 350 psi (see K. C. Kao and J. B. Higham, “The effects of hydrostatic pressure, temperature, and voltage duration on the electric strengths of hydrocarbon liquids,” J. Electrochem. Soc., vol. 108, no. 6, pp. 522-528, June 1961). Pressurized flowing dielectric switches which can switch several hundred kilovolts are known in the art. However, such switches which operate at or near atmospheric pressure require substantial dielectric flow rates of 10-1000 liters per second (l/sec) when they are used to switch multikilojoule pulses. In 1992, subnanosecond rise time, kilohertz rep-rate oil switches were built and demonstrated that could operate at up to 290 kV at 200 pps and at 170 kV with a rep-rate of 1000 pps. The demonstrated rise time into a 97 Ω resistive load was 280 ps. The modulator system, which utilized near atmospheric medium pressure oil switches, transferred a peak energy of 50 J per pulse (R. Curry et al., “The Development and Testing of Subnanosecond-Rise, Kilohertz Oil Switches for The Generation of High-Frequency Impulses”, IEEE Transactions on Plasma Science, Vol. 20, No. 3, June 1992, pp. 383-392, incorporated herein by reference) and demonstrated significant improvement in the breakdown jitter of liquid switches. These oil switches utilized transformer oil at pressures ranging from 1 atmosphere up to 100 psig. The flow rate geometries used in the switches included cross flow, or axial flow in switches that had a near uniform and enhanced electrode geometry. However, these switches were unable to switch kilojoules of energy for they were limited by residual bubbles at a flow rate of 1.6-7.57 l/sec at a repetition rate of over 100 pulses per second (pps).
Single-shot work on high pressure liquid switches examined the effects of pressure upon breakdown voltage (see J. Leckbee, R. Curry, K. McDonald, R. Cravey, and A. Grimmis, “An advanced model of a high pressure liquid dielectric switch for directed energy applications,” in Proc. IEEE 14th Int'l. Pulse Power Conf., 2003, pp. 1389-1393, and J. Leckbee, R. Curry, K. McDonald, P. Norgard, R. Cravey, G. Anderson and S. Heidger, “Design and testing of a high pressure, rep-rate, liquid dielectric switch for directed energy applications,” in Proc. IEEE 26th Int'l. Power Modulator Conf., 2004, pp. 193-196
When a high voltage pulse is applied to a flowing dielectric switch, once the switch breakdown voltage is reached, a streamer is launched and subsequent avalanche ionization and breakdown of the dielectric results. The arc then ionizes the dielectric medium and a gas bubble is formed between the electrodes. As the hydraulic or hydrostatic pressure is increased, the bubble size decreases. It is known that above a critical pressure for certain liquids, no bubbles are formed by charge injection (R. Kattan et al., “Formation of Vapor Bubbles in Non-polar Liquids Initiated by Current Pulses”, IEEE Transactions on Electrical Insulation Vol. 26, No. 4, August 1991, pp 656-662, incorporated herein by reference). However, below a given operating or critical pressure the diameter of the bubble expands well beyond the electrode separation distance. The gas bubble grows and subsequently collapses, oscillating, until it finally rapidly degenerates into both suspended micro-bubbles and discharge byproducts (principally hydrocarbons) that encompass a large volume, if not the entirety, of the switch housing and electrode region.
Liquid dielectric insulated switches cannot sustain high voltages when gas bubbles, dissolved gases, and hydrocarbon byproducts are present because arcing or pre-firing is uncontrollably self-initiated. This also prevents recovery of the switch if voltage were reapplied before the entire volume of liquid in the switch could be exchanged, thus reducing the required achievable repetition rate because of the enormous liquid flow rates that would otherwise be required. Consequently, the repetition rate attainable by present day low-pressure liquid dielectric switches which transfer 100 J-1 MJ is typically limited to much less than one pulse per second, thereby eliminating them from addressing the high average power requirements of many crucial applications. This phenomenon occurs in all known liquid dielectric media suitable for pulse power switching applications, including water, water-glycol solutions, transformer oil, polyalphaolefin (PAO), and other synthetic dielectrics.
Thus there is a need for a kilovolt to megavolt capable, multijoule to megajoule range high power switch with high repetition rate operation, minimized dielectric media flow volume requirements with maximized local flow velocity in the vicinity of the electrodes; minimized electrode erosion; and reduced byproduct formation. There is also need for a compact switch with reduced acoustic impulse and a reduced EMI signature, and with enhanced reliability due to the inhibition of the access and/or adherence of the discharge byproducts to the switch housing solid insulators. The ability of the switch to utilize fluids such as PAO or other synthetic or natural dielectrics that are compatible with existing airframe and aerospace systems is a major advantage, allowing the switch to be integrated with an existing airframe hydraulic system, thereby reducing the volume of support equipment required for directed energy systems.